[PATCH v5 0/2] Implement `git log --merge` also for rebase/cherry-pick/revert

[Philippe Blain <levraiphilippeblain@gmail.com>]

Changes in v5:
- Marked error messages for translation and tweaked them as suggested by Phillip
- Reworded the message of 2/2 as suggested by Phillip
- Removed the change to gitk's doc in 2/2 as pointed out by Johannes
- Fixed the trailers in 2/2
- Improved the doc in 2/2 as suggested by Phillip and Jean-Noël

Changes in v4:
- Added a commit message for 2/2 detailing the use case and summarizing the discussion in the thread
- Adjusted the documentation of the option

---
Michael Lohmann (2):
      revision: ensure MERGE_HEAD is a ref in prepare_show_merge
      revision: implement `git log --merge` also for rebase/cherry-pick/revert

 Documentation/rev-list-options.txt |  7 +++++--
 revision.c                         | 27 +++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 6 deletions(-)
---
base-commit: 186b115d3062e6230ee296d1ddaa0c4b72a464b5
change-id: 20240210-ml-log-merge-with-cherry-pick-and-other-pseudo-heads-05bd8e8797db

Range-diff versus v4:

1:  37405be1a3 ! 1:  c9536431d1 revision: ensure MERGE_HEAD is a ref in prepare_show_merge
    @@ Commit message
         Helped-by: Junio C Hamano <gitster@pobox.com>
         Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
         Signed-off-by: Junio C Hamano <gitster@pobox.com>
    +    Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
     
      ## revision.c ##
     @@ revision.c: static void prepare_show_merge(struct rev_info *revs)
    @@ revision.c: static void prepare_show_merge(struct rev_info *revs)
     +			&oid, NULL))
      		die("--merge without MERGE_HEAD?");
     +	if (is_null_oid(&oid))
    -+		die("MERGE_HEAD is a symbolic ref???");
    ++		die(_("MERGE_HEAD is a symbolic ref?"));
      	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
      	add_pending_object(revs, &head->object, "HEAD");
      	add_pending_object(revs, &other->object, "MERGE_HEAD");
2:  6ac1608809 ! 2:  1641c4be81 revision: implement `git log --merge` also for rebase/cherry-pick/revert
    @@ Commit message
         to merge conflicts also for other mergy operations besides merges, like
         cherry-pick, revert and rebase.
     
    -    For rebases, an interesting range to look at is HEAD...REBASE_HEAD,
    -    since the conflicts are usually caused by how the code changed
    -    differently on HEAD since REBASE_HEAD forked from it.
    +    For rebases and cherry-picks, an interesting range to look at is
    +    HEAD...{REBASE_HEAD,CHERRY_PICK_HEAD}, since even if all the commits
    +    included in that range are not directly part of the 3-way merge,
    +    conflicts encountered during these operations can indeed be caused by
    +    changes introduced in preceding commits on both sides of the history.
     
    -    For cherry-picks and revert, it is less clear that
    -    HEAD...CHERRY_PICK_HEAD and HEAD...REVERT_HEAD are indeed interesting
    -    ranges, since these commands are about applying or unapplying a single
    -    (or a few, for cherry-pick) commit(s) on top of HEAD. However, conflicts
    -    encountered during these operations can indeed be caused by changes
    -    introduced in preceding commits on both sides of the history.
    +    For revert, as we are (most likely) reversing changes from a previous
    +    commit, an appropriate range is REVERT_HEAD..HEAD, which is equivalent
    +    to REVERT_HEAD...HEAD and to HEAD...REVERT_HEAD, if we keep HEAD and its
    +    parents on the left side of the range.
     
    -    Adjust the code in prepare_show_merge so it constructs the range
    -    HEAD...$OTHER for each of OTHER={MERGE_HEAD, CHERRY_PICK_HEAD,
    -    REVERT_HEAD or REBASE_HEAD}. Note that we try these pseudorefs in order,
    -    so keep REBASE_HEAD last since the three other operations can be
    -    performed during a rebase. Note also that in the uncommon case where
    -    $OTHER and HEAD do not share a common ancestor, this will show the
    -    complete histories of both sides since their root commits, which is the
    -    same behaviour as currently happens in that case for HEAD and
    -    MERGE_HEAD.
    +    As such, adjust the code in prepare_show_merge so it constructs the
    +    range HEAD...$OTHER for OTHER={MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD
    +    or REBASE_HEAD}. Note that we try these pseudorefs in order, so keep
    +    REBASE_HEAD last since the three other operations can be performed
    +    during a rebase. Note also that in the uncommon case where $OTHER and
    +    HEAD do not share a common ancestor, this will show the complete
    +    histories of both sides since their root commits, which is the same
    +    behaviour as currently happens in that case for HEAD and MERGE_HEAD.
     
         Adjust the documentation of this option accordingly.
     
    -    Co-authored-by: Philippe Blain <levraiphilippeblain@gmail.com>
         Co-authored-by: Johannes Sixt <j6t@kdbg.org>
    -    Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
    +    Co-authored-by: Philippe Blain <levraiphilippeblain@gmail.com>
         Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
         [jc: tweaked in j6t's precedence fix that tries REBASE_HEAD last]
         Signed-off-by: Junio C Hamano <gitster@pobox.com>
    -
    - ## Documentation/gitk.txt ##
    -@@ Documentation/gitk.txt: linkgit:git-rev-list[1] for a complete list.
    - 
    - --merge::
    - 
    --	After an attempt to merge stops with conflicts, show the commits on
    --	the history between two branches (i.e. the HEAD and the MERGE_HEAD)
    --	that modify the conflicted files and do not exist on all the heads
    --	being merged.
    -+	Show commits touching conflicted paths in the range `HEAD...$OTHER`,
    -+	where `$OTHER` is the first existing pseudoref in `MERGE_HEAD`,
    -+	`CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
    -+	when the index has unmerged entries.
    - 
    - --left-right::
    - 
    +    Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
     
      ## Documentation/rev-list-options.txt ##
     @@ Documentation/rev-list-options.txt: See also linkgit:git-reflog[1].
    @@ Documentation/rev-list-options.txt: See also linkgit:git-reflog[1].
      --merge::
     -	After a failed merge, show refs that touch files having a
     -	conflict and don't exist on all heads to merge.
    -+	Show commits touching conflicted paths in the range `HEAD...$OTHER`,
    -+	where `$OTHER` is the first existing pseudoref in `MERGE_HEAD`,
    ++	Show commits touching conflicted paths in the range `HEAD...<other>`,
    ++	where `<other>` is the first existing pseudoref in `MERGE_HEAD`,
     +	`CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
    -+	when the index has unmerged entries.
    ++	when the index has unmerged entries. This option can be used to show
    ++	relevant commits when resolving conflicts from a 3-way merge.
      
      --boundary::
      	Output excluded boundary commits. Boundary commits are
    @@ revision.c: static void add_pending_commit_list(struct rev_info *revs,
     +				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
     +				oid, NULL)) {
     +			if (is_null_oid(oid))
    -+				die("%s is a symbolic ref???", other_head[i]);
    ++				die(_("%s is a symbolic ref?"), other_head[i]);
     +			return other_head[i];
     +		}
     +
    -+	die("--merge without MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD or REBASE_HEAD?");
    ++	die(_("--merge requires one of the pseudorefs MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD or REBASE_HEAD"));
     +}
     +
      static void prepare_show_merge(struct rev_info *revs)
    @@ revision.c: static void prepare_show_merge(struct rev_info *revs)
     -			&oid, NULL))
     -		die("--merge without MERGE_HEAD?");
     -	if (is_null_oid(&oid))
    --		die("MERGE_HEAD is a symbolic ref???");
    +-		die(_("MERGE_HEAD is a symbolic ref?"));
     -	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
     +	other_name = lookup_other_head(&oid);
     +	other = lookup_commit_or_die(&oid, other_name);